Hydroforming is a well known process for forming metal workpieces by means of pressurized fluid. It is performed using high fluid pressures that are applied internally—such as by internal hydroforming of tubes or pipes—or externally—such as by hydromechanical forming or flexforming of sheet metal—to workpieces that are mostly relatively thin-walled and that are positioned in a tool. In the internal hydroforming the conventional tools consist of upper and lower tool halves and/or tool holder halves that are vertically movable relative to each other. The tool halves carry upper and/or lower dies respectively, and are relatively movable between an open position for loading blanks and for unloading processed workpieces, and a closed position in which the fluid pressure is applied to the blank. Generally speaking the blank is formed by forcing it into contact with the wall or walls of a hollow space or cavity formed by the die or dies between the tool halves. In the flexforming process the upper tool half/tool holder is replaced by a plate holder which through a flexible membrane closes the die cavity formed in the lower tool and which forms a pressure fluid space above the membrane and the blank. However, in this specification this type of plate holder for the flexforming process will likewise be generally referred to as an upper tool half.
The very high fluid pressures applied to the workpiece for the forming operation generate extreme outwardly directed forces acting to push the dies and thereby the tools apart. The fluid pressure applied to the workpiece, and thus also to the tool halves, from within the hollow space formed by the dies, is often in the order of several thousand bars. The resulting tool separating forces are likewise extremely high, and will for most applications amount to several thousand kN. Obviously it is vital for the hydroforming process that the tool halves and specifically the dies are securely closed and maintained in their mutual position during the entire forming process. In the conventional apparatus for performing hydroforming a powerful press provides the extreme closing forces. Normally the press ram carries the upper tool half and the lower tool half is stationary and supported on a machine base.
The conventionally employed press may be required to produce a closing force of up to about 100 000 kN and is in any event, even for hydroforming processes for smaller workpieces, extremely expensive and requires a great amount of space. Naturally, the very high cost of the press and the complexity of the hydroforming apparatus, as such, make the total investment cost unbearable to many small and middle size companies, and thereby limits the availability of the hydroforming processes. In practice the conventional technique will therefore only be available to larger companies. The hydroforming techniques have exceptional advantages over many traditional forming techniques, and therefore there is a general need within this field for improvements that may reduce the complexity and cost of the equipment for hydroforming and that may simplify the process.
Although the above discussed problems and circumstances are emphasized within the field of hydroforming they do indeed also exist within other technical fields, such as injection molding. Therefore the need for improved technical solutions eliminating tool closing and holding problems is not restricted to said hydroforming field, but applies to any molding or forming technique working with raised internal pressures.